At first glance, Louisiana oilman Greg Fleniken's death looked to police like it was from natural causes.

"That's what we had just figured it must be at the scene," Beaumont Police detective Scott Apple told ABC News' "20/20."

On Sept. 15, 2010, Fleniken walked into his room 348 in the Elegante Hotel in Beaumont, Texas, but he never checked out.

Hours later, Beaumont police found 55-year-old Fleniken dead on the floor, with the television still on, the air conditioner off, and more than $1,000 of Fleniken's cash still in the room. Candy bars, soda, and cigarettes were near the bed.

In his initial investigation, detective Apple learned that Fleniken, a travelling businessman, was a creature of habit, staying at home in Louisiana on the weekends and staying at the Elegante Hotel during the week.

"He'd set up, and that was his routine for the evenings," Apple said. "He smoked a lot, and he had his candy bars, because he had a sweet tooth."

Fleniken was successful in the oil business and known to have many friends. "He'd give you...not only the shirt off his back, but the shoes off his feet," said Martin of his friend.

According to Apple, Fleniken's body was immediately taken away to the medical examiner to essentially confirm that his cause of death was a heart attack.

But medical examiner Dr. Tommy Brown found that the body suffered severe internal damage, with broken ribs, lacerations to the liver and scrotum, and a hole in the heart, all caused, according to Brown, by blunt force trauma.

Brown ruled Fleniken's death a homicide.

Apple worked tirelessly to find someone in Beaumont with the means, motive and opportunity to commit the crime. Aside from a circuit breaker in Fleniken's room needing to be fixed that night, nothing unusual happened at the hotel. Apple questioned the nearby hotel guests, electricians from Wisconsin who were working in one of the refineries in the area.

Fleniken's widow Susie Fleniken was desperate for answers and hired private investigator Ken Brennan, a former New York police officer and Drug Enforcement Administration agent, now living in Florida.

"She wanted to see if I could find out who killed her husband," Brennan said.

After meeting with Susie Fleniken to eliminate her as a suspect in her husband's death, Brennan went to Beaumont, Texas, and met with detective Apple to work on the investigation.

"It isn't like I need the police to do an investigation, but it certainly makes the job a hell of a lot easier," Brennan said.

Brennan and Apple began by reviewing the case file, going through the hotel room and even taking a closer look at the victim.

They eliminated the possibility of a robbery, since Fleniken's money was still in the room, and after checking out Fleniken's co-workers at the oil company, they found he had no enemies. The handyman who came to Fleniken's room to fix his circuit breaker also had an air-tight alibi.

Brennan and Apple only had the hotel's security camera footage, crime scene photographs, and the knowledge of the broken circuit, until Brennan had a breakthrough that he shared with Apple.

"I said, 'I think I know how this guy died,'" Brennan recalled.

"I said, 'I think I know when it happened...I think I know who killed him, and I think I know how we're going to catch him.'"